How do sails work?

[Thistle]

It doesn't help this bit:


Because there is literally no apparent wind in the scenario they fail to correctly describe.

Please, if you can't provide help at the level requested don't spend any more of your valuable time on this thread. Thanks.

 

[Buck Turgidson]

OK,
In the simplest terms I can think of, given that I hope all sailors understand the concept of apparent wind.

They go fast because they sail in very high apparent winds due to the very low drag their foils create.


It's exactly the same as a foiling moth, those little boats sail over 30kts because almost all of the power the sail makes is used as thrust.

The boat doesn't fall over because in the case of a moth the sailor is creating a righting force and in the case of the AC boats one keel is creating lift and the other is sticking out like the sailor of a moth.

 

[Thistle]

So it's all about reducing drag rather than anything particularly special about the sails? (Though, no doubt, a better sail shape will give a boat the edge over an otherwise identical competitor.) Reduce drag and, with the same energy/wind input, you'll get higher speed. As speed increases the apparent wind goes forward and you sheet in to maintain the optimum angle of attack. Eventually you get to a speed where you can be close-hauled on a broad reach?

 

[RJJ]

So it's all about reducing drag rather than anything particularly special about the sails? (Though, no doubt, a better sail shape will give a boat the edge over an otherwise identical competitor.) Reduce drag and, with the same energy/wind input, you'll get higher speed. As speed increases the apparent wind goes forward and you sheet in to maintain the optimum angle of attack. Eventually you get to a speed where you can be close-hauled on a broad reach?

Yep. It's very obvious if you look at photos of various three-sail dinghies on a broad reach in a force 3. The wayfarer is at hull speed with pole back and boom near the shroud. The 420 is similar. The 505 is fully-planing and, for the same heading, has the sails sheeted in / pole forward a little. At RS700 speed, the mainsail is just inside the transom. An 18 footer or 49er has the sail sheeted just inches from close-hauled.

Think of it as two wings, one above and one below the water. Both wings are generating lift and drag (in the horizontal sense); you want as much of the former as possible and to minimise the latter. The hull, traditionally, was just a low-drag flotation device (the accommodation potential is incidental to sailing).

The Bible is Bethwaite's High Performance Sailing. I didn't get that into the maths, but the descriptions of the progress of 18 footers through the C20 is brilliantly presented and highly digestible.

 

[Buck Turgidson]

So it's all about reducing drag rather than anything particularly special about the sails? (Though, no doubt, a better sail shape will give a boat the edge over an otherwise identical competitor.) Reduce drag and, with the same energy/wind input, you'll get higher speed. As speed increases the apparent wind goes forward and you sheet in to maintain the optimum angle of attack. Eventually you get to a speed where you can be close-hauled on a broad reach?

mmm
The sails are extremely special. They are somewhere between normal single piece of cloth that you or I use and the solid wing sail of previous AC boats.
They have a rotating mast as the leading edge and twin skins which create very aerodynamic profiles. This leads to much higher lift to drag ratios than our plain old sails.

 

[JumbleDuck]

"True wind always pushes a boat. If a boat sails absolutely perpendicular to true wind, so the sail is flat to the wind and being pushed from behind, then the boat can only go as fast as the wind—no faster. "

Seems OK to me. The point about things which go downwind faster than the wind - or which go directly into wind, which is the same thing, upside down and backwards - is that they all use apparent wind which is greater than than the true wind, either by tacking downwind or by whirling the "sails" round an axle - which is also the same thing.

if you really want to confuse yourself, try this for size. What the wikipedia article doesn't tell you is that there was massive controversy - people familiar with sailing, land yachts and physics wrote this off as a prank and said the trials were faked; some still insist it's a fake fourteen years later.

We had a massive thread about it at the time, with stupid and ignorant people persisting in believing it couldn't be true because it isn't possible or some such nonsense. Hint: the flow of mechanical power is from the road wheels to the airscrew.

I hate willy waving and argument from authority, but just for once I'd like to mention that I am a lecturer - a senior lecturer, no less - in mechanical engineering at a major UK university and I earn my living largely by teaching fluid dynamics, including the theory of aerofoils and lift. Trust me, I have heard every misguided argument there is.

 

[JumbleDuck]

That's the simplest expression of lift. If you want to understand how they sail so fast, that is I'm sorry to say, where you have to start.
At a Cl of 1 and 18kt apparent wind the mainsail makes about 7600 Newtons of force.
If that is enough to accelerate to 20kt apparent you will now be generating 9400 Newton's of force.
And at 32kt 30400Newtons.

Sorry, no, because as you speed up you decrease the angle of attack and therefore the lift coefficient. You can deal with this a bit by sheeting in, but there are limits to that.

Of course the formula for induced drag is the same so at some point the gains become marginal as you need lots of lift for righting moment which in turn makes lots of induced drag.

Sorry, no. Induced drag is inversely proportional to the square of wind speed and so reduces as you speed up. You are probably thinking of profile drag, which does indeed increase with the square of the wind speed.

 

[JumbleDuck]

That may be fine if you understand the terms (CI, ρ , V, S) and want to quantify the result. I just want a superficial understanding of how a boat can sail upwind and / or downwind at speeds greater than the windspeed. The article linked to by Euphonyx may not be perfect but I find it more helpful at my current level of understanding.

Can I try?

Think of an ice yacht sitting stationary on the ice with the wind coming exactly abeam. The sailor (pilot?) sheets in, the sail starts to generate lift (which is at right angles to the apparent wind, by definition) and so the yacht starts to move forwards.

As it does it generates its own wind from dead ahead and adding this to the wind coming from one side swings the apparent wind round to the front and increases it a bit. If the true wind is 20 kt from the side, a speed of 20 kt over the ice generates another 20 kt from dead ahead and the combination is 28 kt from 45 degrees off the bow.

However, there is nothing magic about that angle. The sail is still generating lift and some of that lift is still directed forwards A smaller proportion that before, because "right angles to the apparent wind" is now 45 degrees of the other bow instead of dead a head, so some of the lift is trying to tip the ice yacht over. However, if the sail is big enough and the friction and drag are low enough there will still be an excess of forward force and so the ice yacht can continue to accelerate until ...

Until either the forward component of lift exactly balances the ice friction and other drag or the sail is sheeted in as far as it can be, which in the best case is probably about 10 degrees of the centreline with the apparent wind about 20 degrees off. At that point the sideways component of wind is still 20 kt but the forward component is about 55kt, which is the theoretical limit. I don't how fast ice yachts actually go but I'd be surprised if it was this fast. Twice the wind speed seems more reasonable.

It's much harder to do this (and by "much harder" I means "effectively impossible) in a water yacht because the friction is so much higher ... unless you are on planing or hydrofoils ...

 

[Thistle]

Can I try?

Think of an ice yacht sitting stationary on the ice with the wind coming exactly abeam. The sailor (pilot?) sheets in, the sail starts to generate lift (which is at right angles to the apparent wind, by definition) and so the yacht starts to move forwards.

As it does it generates its own wind from dead ahead and adding this to the wind coming from one side swings the apparent wind round to the front and increases it a bit. If the true wind is 20 kt from the side, a speed of 20 kt over the ice generates another 20 kt from dead ahead and the combination is 28 kt from 45 degrees off the bow.

However, there is nothing magic about that angle. The sail is still generating lift and some of that lift is still directed forwards A smaller proportion that before, because "right angles to the apparent wind" is now 45 degrees of the other bow instead of dead a head, so some of the lift is trying to tip the ice yacht over. However, if the sail is big enough and the friction and drag are low enough there will still be an excess of forward force and so the ice yacht can continue to accelerate until ...

Until either the forward component of lift exactly balances the ice friction and other drag or the sail is sheeted in as far as it can be, which in the best case is probably about 10 degrees of the centreline with the apparent wind about 20 degrees off. At that point the sideways component of wind is still 20 kt but the forward component is about 55kt, which is the theoretical limit. I don't how fast ice yachts actually go but I'd be surprised if it was this fast. Twice the wind speed seems more reasonable.

It's much harder to do this (and by "much harder" I means "effectively impossible) in a water yacht because the friction is so much higher ... unless you are on planing or hydrofoils ...

Many thanks. A very clear and understandable answer.

 

[Buck Turgidson]

Sorry, no, because as you speed up you decrease the angle of attack and therefore the lift coefficient. You can deal with this a bit by sheeting in, but there are limits to that.



Sorry, no. Induced drag is inversely proportional to the square of wind speed and so reduces as you speed up. You are probably thinking of profile drag, which does indeed increase with the square of the wind speed.

On point 1 I specified a Cl of 1 as sheeting in is exactly why the angle of attack doesn't need to change and why we have seen AC boats with the "boom" on the windward side at times.

Point 2, Yes and no.
for an aircraft in level flight you are correct but that is because lift remains constant. For a boat the lift is thrust and therefore must increase hence no reduction of Alpha with velocity increase and therefore no reduction in induced drag.

 

[TLouth7]

I don't how fast ice yachts actually go but I'd be surprised if it was this fast. Twice the wind speed seems more reasonable.

It's much harder to do this (and by "much harder" I means "effectively impossible) in a water yacht because the friction is so much higher ... unless you are on planing or hydrofoils ...

Ice yachts go up to about 6x true wind speed. AC75s go at least 3x true windspeed, with VMGs of around 2.

Great explanation by the way.

 

[RJJ]

Can I try?

Think of an ice yacht sitting stationary on the ice with the wind coming exactly abeam. The sailor (pilot?) sheets in, the sail starts to generate lift (which is at right angles to the apparent wind, by definition) and so the yacht starts to move forwards.

As it does it generates its own wind from dead ahead and adding this to the wind coming from one side swings the apparent wind round to the front and increases it a bit. If the true wind is 20 kt from the side, a speed of 20 kt over the ice generates another 20 kt from dead ahead and the combination is 28 kt from 45 degrees off the bow.

However, there is nothing magic about that angle. The sail is still generating lift and some of that lift is still directed forwards A smaller proportion that before, because "right angles to the apparent wind" is now 45 degrees of the other bow instead of dead a head, so some of the lift is trying to tip the ice yacht over. However, if the sail is big enough and the friction and drag are low enough there will still be an excess of forward force and so the ice yacht can continue to accelerate until ...

Until either the forward component of lift exactly balances the ice friction and other drag or the sail is sheeted in as far as it can be, which in the best case is probably about 10 degrees of the centreline with the apparent wind about 20 degrees off. At that point the sideways component of wind is still 20 kt but the forward component is about 55kt, which is the theoretical limit. I don't how fast ice yachts actually go but I'd be surprised if it was this fast. Twice the wind speed seems more reasonable.

It's much harder to do this (and by "much harder" I means "effectively impossible) in a water yacht because the friction is so much higher ... unless you are on planing or hydrofoils ...

thanks. Can I ask another subtle question?

Fully understand why higher-performing sailcraft (on land, sea or ice) generate ever-narrower apparent wind angles as they accelerate, both upwind and down.

So why is VMG optimised at such a narrow envelope of TWA? Broadly, all these very different boats point at about 40 degrees to the true wind, sailing close-hauled. Those with more efficient grip on the land, sea or ice get to make less leeway, but from a tubby trailer-sailer up to an AC75 (in flat water and a moderate design wind, say 10-12 knots) everyone's still sailing within 5 degrees of each other.

Enjoying this thread BTW.

 

[TLouth7]

To understand how a boat can sail close hauled or on a broad reach with VMG faster than the wind, take JumbleDuck's explanation in post #28 and instead of starting your boat at 90 degrees to the true wind, imagine it starting at 45 or 135 degrees. The boat can keep accelerating on this course until either it can't sheet in any more, or the drag is higher than the thrust. For a boat with narrow sheeting angles and low drag (foiler) this limit can be at a VMG greater than the windspeed.

So it's all about reducing drag rather than anything particularly special about the sails?

Remember that a low drag sailplan allows you to sail closer to the apparent wind before then net force on the sails stops acting forwards. A sail with good lift to drag ratio will have its force arrow (Ft in the image below) closer to perpendicular to the sail. The AC75 boats have a few features to make the sail low drag. Firstly they have wingmasts that rotate, so there is not a big circular blob spoiling the front of the airfoil shape. Secondly they have two fabric skins which means there is a smooth flow from the back edges of the mast to the sail itself. Thirdly the sails are very tall; in general a high aspect ratio (tall and narrow) foil has lower drag than a dumpy one. Fourthly the sails are flush to the deck at their foot. This stops air flowing from the windward to leeward side and so improves the lift (or reduces drag depending on which way you look at it).

 

[RJJ]

mmm
The sails are extremely special. They are somewhere between normal single piece of cloth that you or I use and the solid wing sail of previous AC boats.
They have a rotating mast as the leading edge and twin skins which create very aerodynamic profiles. This leads to much higher lift to drag ratios than our plain old sails.

We mortals have to cope with a thin cloth sail; we try to approximate to perfect aerofoil geometry with the major constraint that it has to work on both tacks.

The differences, if I understand correctly, are with AC75

1. they can make the inner surface flatter and the outer surface more rounded.
2. the sails are almost perfectly faired into the side profile of the mast

...which means they are approaching Nirvana of an perfect aerofoil section, and can replicate it on both tacks.

 

[JumbleDuck]

Ice yachts go up to about 6x true wind speed. AC75s go at least 3x true windspeed, with VMGs of around 2.

Thanks. My fault - I was still thinking of a beam wind. Would 3x windspeed be about right for that? Of course a higher multiple is possible for wind behind the beam.

Great explanation by the way.

Merci beaucoup. "Can an ice yacht sail faster than the wind?" was one of the questions at my admissions interview for university back in 19mumble.

 

[JumbleDuck]

We mortals have to cope with a thin cloth sail; we try to approximate to perfect aerofoil geometry with the major constraint that it has to work on both tacks.

The differences, if I understand correctly, are with AC75

1. they can make the inner surface flatter and the outer surface more rounded.
2. the sails are almost perfectly faired into the side profile of the mast

...which means they are approaching Nirvana of an perfect aerofoil section, and can replicate it on both tacks.

As a matter of interest, why are they back on cloth sails this time? I had assumed that wings would be here to stay.

 

[Buck Turgidson]

As a matter of interest, why are they back on cloth sails this time? I had assumed that wings would be here to stay.

There is a north sails video which claimed it was a conscious decision to make the technology more transferable. I imagine the truth is they were sick of the faffing around removing the solid wing.

 

[TLouth7]

Thanks. My fault - I was still thinking of a beam wind. Would 3x windspeed be about right for that? Of course a higher multiple is possible for wind behind the beam.

In theory a beam reach wrt true wind should give the highest multiple of true windspeed. This is because it gives the largest AWA for a given ratio.

As a matter of interest, why are they back on cloth sails this time? I had assumed that wings would be here to stay.

They wanted to avoid the need for a crane every time the boat starts or stops sailing.

 

[Buck Turgidson]

To understand how a boat can sail close hauled or on a broad reach with VMG faster than the wind, take JumbleDuck's explanation in post #28 and instead of starting your boat at 90 degrees to the true wind, imagine it starting at 45 or 135 degrees. The boat can keep accelerating on this course until either it can't sheet in any more, or the drag is higher than the thrust. For a boat with narrow sheeting angles and low drag (foiler) this limit can be at a VMG greater than the windspeed.


Remember that a low drag sailplan allows you to sail closer to the apparent wind before then net force on the sails stops acting forwards. A sail with good lift to drag ratio will have its force arrow (Ft in the image below) closer to perpendicular to the sail. The AC75 boats have a few features to make the sail low drag. Firstly they have wingmasts that rotate, so there is not a big circular blob spoiling the front of the airfoil shape. Secondly they have two fabric skins which means there is a smooth flow from the back edges of the mast to the sail itself. Thirdly the sails are very tall; in general a high aspect ratio (tall and narrow) foil has lower drag than a dumpy one. Fourthly the sails are flush to the deck at their foot. This stops air flowing from the windward to leeward side and so improves the lift (or reduces drag depending on which way you look at it).

View attachment 109566

Need the foil in there to get an idea of where the thrust comes from.

 

[TLouth7]

Need the foil in there to get an idea of where the thrust comes from.

I was trying to express why a high lift to drag ratio on the sail is useful, rather than provide a full free body diagram of all the forces on a boat.

The same argument applies to boat keel/daggerboards. They must provide sufficient sideways force to counteract the sideways component of the force on the sail. If they can do this with minimal drag (deep, narrow foil) then there is less total drag on the boat which is good for going fast.